The present invention relates to electric discharge and incandescent lamps of the type comprising a light-transmitting envelope having contained therein a source of light such as a filament or a pair of electrodes between which an electric discharge is produced, such that on energisation of the lamp there will be emitted visible light, I.R. and U.V. radiation, and incorporating at least one seal of solid light-transmitting material in which is embedded at least one current-supply foil running substantially the length of the seal, the current-supply foil being connected at one end to a conducting support for a filament or for an electrode within the envelope, and at the other end to a current-supply pin. The invention is particularly appplicable to high power (e.g. 2.5 kW) electric discharge lamps such as the MEI lamp, which are used for television and film lighting. Such lamps comprise a silica envelope integral with either one or two silica supporting stems providing one or two said seals and are referred to as single and double-ended lamps respectively, the current-supply foils being generally of molybdenum. The protruding end of each current supply pin is usually connected to a metal lamp cap which encloses the outer end of the seal.
The adjacent ends of the filament or electrode supports and the current-supply pins are also embedded in the material of the seal, but as the current-supply pins do not form a completely air-tight seal with the surrounding silica, the welded ends of the foils are liable to oxidise and eventually break if they become hotter than about 250.degree. C. In fact, tests have shown that in a 2.5 kW MEI discharge lamp provided with cylindrical seals 110 mm long and 15 mm in diameter burning in a luminaire in still air, the temperature of the current-supply pins can reach 370.degree. C. if conventional lamp caps are used. Even when the capped lamp is run in a lampholder fitted with cooling fins the temperature of the pins can reach 320.degree. C. Thus even when relatively long seals are provided, the outer ends of the foils can become sufficiently hot for oxidation to occur.
Attempts have been made to reduce the amount of heat reaching the outer ends of the foils by locating a flat collar around the outer end of the or each stem of a lamp envelope embodying a foil seal between the or each welded end of the foil and the lamp envelope, as described in our U.S. patent application Ser. No. 456,273 filed Jan. 6, 1983, now abandoned, and co-pending U.S. patent application Ser. No. 804,096 filed Dec. 3, 1985. The collar acts as an external heat shield, shielding the end of the foil from heat and light radiated from the lamp envelope. A heat sink is commonly fitted to the lamp cap or current-supply pin to increase the heat dissipation from the seal. Even when such precautions are taken, it has been found necessary in some cases to make the stems much longer than is necessary to obtain effective seals. It has been found that this is largely due to heat and light emanating from the source of light in the lamp envelope reflected and transmitted internally through the solid material of the seals towards the outer end of the foils. A considerable proportion of this heat and light which reaches the outer ends of the foils is internally reflected from the surfaces of the seals, which thereby acts as "light pipes". The present invention provides a particularly simple method of reducing the "light pipe" effect in lamps of the type specified.